S-8333CAAA-I8T1U Datasheet, PDF (17/44 Pages) Seiko Instruments Inc – PWM CONTROL SWITCHING REGULATOR CONTROLLER

STEP-UP, FOR LCD BIAS SUPPLY, 1-CHANNEL, PWM CONTROL SWITCHING REGULATOR CONTROLLER
Rev.4.2_01
S-8333 Series
5. Oscillation frequency and maximum duty ratio setting resistors (ROSC, RDuty)
With the S-8333 Series, the oscillation frequency can be set in a range of 280 kHz to 1.08 MHz using external
resistance. Connect a resistor across the ROSC and VSS pins. Select the resistor by using the following
equation and referring to Figure 12. However, the following equation and figure assume that the resistance
value is the desired value and show the theoretical values when the IC is in the typical conditions. Note that
fluctuations of resistance and IC are not considered.
R_OSC [kΩ] ≅
130 × 10³
f_OSC [kHz]
1400
1200
1000
800
600
400
200
0
0
200
400
600
ROSC [kΩ]
Figure 12 ROSC vs. fOSC
With the S-8333 Series, the maximum duty ratio can be set in a range of 47% to 88.5% (between 47 to 80%, if the
oscillation frequency is less than 500 kHz) by an external resistor. Connect the resistor across the RDuty and
VSS pins. Select the resistance by using the following equation and referring to Figure 13. The maximum duty
ratio fluctuates according to the oscillation frequency. If the value of ROSC is changed, therefore, be sure to
change the value of RDuty so that it is always in proportion to RDuty / ROSC. However, the following equation
and figure assume that the resistance value is the desired value and show the theoretical values when the IC is
in the typical conditions. Note that fluctuations of resistance and IC are not considered.
Caution Set max. duty 80% or less if the oscillation frequency is less than 500 kHz.
R_Duty (95.5 − MaxDuty)
R_OSC ^â‰
15.0
100
90
80
70
60
50
40
0
1
2
3
4
RDuty / ROSC
Figure 13 RDuty / ROSC vs. MaxDuty
Connect resistors ROSC and RDuty as close to the IC as possible.
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